ABSTRACT

Annual fluxes, flow-weighted concentrations, and linear least squares trendline calculations for a number of long-term Mississippi River Basin (MRB) sampling sites covering 1981 through 2007, whilst somewhat "noisy," display long-term patterns of decline. Annual flow-weighted concentration plots display the same long-term patterns of decline, but are less noisy because they reduce/eliminate variations due to interannual discharge differences. The declines appear greatest in the middle MRB, but also are evident elsewhere. The pattern for the lower Ohio River differs, and may reflect ongoing construction at the Olmsted lock and dam that began in 1993.

The "Great Flood of 1993" appears to have superimposed a step function (a sharp drop) on the long-term rate of decline in suspended sediment concentrations (SSC), annual fluxes, and flow-weighted concentrations in the middle MRB at St. Louis and Thebes, Missouri, and Vicksburg, Mississippi, and in the lower MRB at St. Francisville, Louisiana. Evidence for a step function at other sites is less substantial, but may have occurred. The step function appears to have resulted from losses in available (erodible) sediment, rather than to a reduction in discharge; hence, the MRB appears to be supply limited rather than discharge limited. These evaluations support the need for daily discharge and SSC data collections in the MRB to better address questions regarding long-term trends in sediment-related issues. This is apparent when the results for the Mississippi River at Thebes and St. Louis sites are compared with those from other MRB sites where intensive (daily) data collections are lacking.

First posted August 28, 2009